Ultra high frequency resonant cavities



Aug. 13, 19456. s. A. SCHELKUNOFF ULTRA-HIGH FREQUENCY RESONANT CAVITIES Filed March 5, 1941 INVENTOR S. A. SCHELKUNOFF ATTORNEK Patented Aug. 13, 1946 ULTRA HIGH FREQUENCY RESONAN T CAVITIES Sergei A. Schelkunoff, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 5, 1941, Serial No. 381,799

Claims. i

This invention relates to novel and convenient forms of ultra-high frequency resonant cavities adapted for use in electron velocity variation systems and the like.

This application is a continuation in part of my copending application, Serial No. 308,376, filed December 9, 1939, relating to high frequency tanks and resonant cavities.

Rigorously exact mathematical methods for computing resonant frequencies of cavities are available for only a few simply shaped cavities such as cavities delimited by right circular cylindrical and by spherical surfaces. Even for cavities bounded by spheroids the computations would be extremely laborious since appropriate tables of functions are not available.

However, as will be demonstrated in the course of the following description, for structures of the general type exemplified by the continuously varying transmission line, the fundamental, or lowest,

resonant frequency can be computed with a degree of accuracy entirely sufiicient for the majority of practical applications. A number of forms of resonant devices of this type are extremely convenient in mechanical form.

It is an object of the invention therefore to provide a new. class of resonant cavities and to teach the principles in accordance with which,

they may be proportioned.

It is a further object to provide convenient methods of determining the fundamental resonant frequencies of cavities bounded by spheroidal, and the like, surfaces.

Other objects will become apparent during the course of the following description of specific illustrative embodiments in connection with the accompanying drawing in which:

Fig. 1 is a diagrammatic representation of a continuously variable electrical transmission line employed to explain the nature of devices of the invention;

Fig. 2 shows in cross-sectional view a structure having a cavity defined by a cylinder and end discs which structure embodies certain characteristic features in common with structures of the invention;

Fig. 3 shows in cross-sectional view a resonant structure having a cavity defined by two concentric hemispheres and a plane ring joinin their bases;

Fig. 4 shows in a cross-sectional view a structure having a cavity defined by two concentric spheres; and

Figs. 5 and 6 show in cross-sectional view structures having cavities definedby two concentric spheroidal members.

In more detail in Fig. 1 a transmission line comprising conductors II and I2, having a length 1, input terminals I6, output terminals 18 and being uniformly variable in impedance along its length by virtue of a uniform variation of distance between the conductors, is shown. The variable distance X employed in the mathematical treatment of the structure hereinunder is measured from the open end of the line as shown in Fig. 1.

The line of Fig. 1 is shown short-circuited at its right end and open-circuited at its left end.

The structur of Fig. l is the electrical analogue of resonant cavities which are characterized, for example, by a uniform variation of capacity between axial positions suitable for the application of electrical excitation and boundary surfaces thereof which correspond to the short-circuited end of the line of Fig. 1. i

' Forms illustrative of those which cavities of this type may take, are illustrated in Figs. 2 to 6, inclusive. I

In Fig. 2 a structure is shown in which the cavity is defined by two discs 20 joined at their outer peripheries by a, cylindrical member 22. The input terminals are the points at which the axial orifices 28 arelocated. and the cylinder is the short circuit applied to the other end of the transmission line. The two parallel discs, of course, comprise a disc transmission line, the properties of which are explained in detail in my copending application, Serial No. 278,032, filed June 8, 1939, now Patent No. 2,235,506, dated March 18, 1941. The cavity is designed to be resonant at a particular predetermined frequency as will be presently explained when a velocity varied stream of electrons is directed along the path of axis 24.

In Fig. 3 a structure is shown in which the cavity is defined by two hemispheres 32 and 34 joined at their bases by plane ring member 36.

The cavity is designed to be resonant at a particular predetermined frequency, as will be presently explained, when a velocity varied stream of electrons is directed along the path of axis 30.

A plurality of the devices of either Fig. 2 or Fig. 3, or some of each, may, of course, be arranged with their orifices on a common axis so that all will respond to excitation by a singl electron stream. In som systems .it may be advantageous to proportion successive devices or groups of devices, so coaxiaily arranged, to beresonant at different frequencies so that response at a num- 3 her of different frequencies will result on the part of some of the devices.

In Fig. 4 two spheres 48 and 42 are concentrically arranged with orifices 48 arranged along a common axis 46 to providea rectilinear path along which an electron stream may be directed. Members 44 are made of insulating material and serve to hold the inner sphere 42' concentrically with respect to the outer sphere 48. The cavity between the two spheres is evidently the equiv-1 alent of two cavities as defined by the. devic of. Fig. 3 placed base to base with member 36 of; Fig. 3 omitted.

In some instances it will bedesirableto proportion the device of Fig. 4 so that the fundamental resonance of the cavity within inner sphere 4-2 is substantially separated in frequency from. the fundamental resonance of the cavity between sphere 40 and 42. fundamental resonances just mentioned may advantageously be identical or nearly so, toreenforce or supplement each other; in their respective reactions upon the electron stream.

Thedevices of Figs. 5 andfi are similar to. that of; 4, except; that two ovoid. or spheroidal members. are substituted, for the spheres of Fig.4..

In Fig. 5 spheroidal member 60 and 62 have orifices, aligned along, the major axis 66. Membars: 64 are spacers of insulating material- In; Fig. 6: members 50 and 52 have orifices aligned along the minor axis 55. Members 5.5' are spacersofinsulatingmaterial;

. Figs; 5 and 6 illustrate that by merely changing the; axis of. excitation agiven device at the invention. may be; made torespondto a different frequency as will become evident, hereinunder.

In explanation of the theory underlying devices ofthe invention the transmission line. of.

Fig. 1: will now be analyzed. At its left-hand terminals it, this line is open-circuited while at its right-hand terminals. [8 the line is short-circuited. by the conductor Id. The voltage. across the short-circuited end is, of course, ubstantially zero. The.- voltage across the open-circuited end of the line is maximum. at the fundamental resonant frequency of the line.

If C be the capacity per unit: length of the line and k the Wave-length of the; fundamental; resonant frequency then, with a degree of accuracy suflicient for the majority of practical applications, the following relationv obtains and Z=-length oflinehe: above'relations are remarkably accurate for structures in which C varies continuously isfy the equation 21d Ju( 0;

Since: the. first root of thisv equation is 2.40;

from Equation 3, \=2.62 checking the value obtained from Formula 1. One. structure having the capacity C. vary in proportion. to the distance;

In other instances: the two 4 from the open end of the line is that shown in Fig. 2 where the radius of the discs is Z. Along the axis 24 the voltage E is maximum at the fundamental resonant frequency of the device and at the peripheries of the discs 20 (short-circuited by cylindrical member 22) the voltage is substantially zero, 0 that the electrical conditions are as described above for the structure of'Fig. 1.

In the case of the structure of Fig. 3 the ca- 'pacity C is proportional to a sin 2 p a where a: is the distance from the driving point along the. arc of the median sphere between the spherical surfaces; From the above formulae, for the case of Fig. 3,

This checks with resultsobtained by J. J-.,Thornpat. page 375. in. the book entitled Recent Re.

be evaluated graphically or numerically. Thus it is obvious that the above method. permits the computation of resonant wave-lengths of cavities ofv general shapes such as. those exemplified; by Figs. 5 and- 6.. The latter structures facilitate control of the distance between the two pathsacross the inter-spheroidal cavity and, therefore lend. themselves. particularly Well for use in electron stream. coupling systems.

The above-described arrangements are, illustra;

tive of the principles of the invention. Numerous other arrangements and, structures Within, the.

spirit. and scope of the invention. will occur to those skilled in the art and no attempt has. here been made to exhaustively cover all possible structures. The. scope of the invention is defined in the following claims;

What is claimed is:

1. In an ultra-high frequency system, aresonant device comprising solely two parallel spheroidal conducting members, and electroconductively insulating members, having highelectrical impedance to the frequencies of the system, said last-stated members serving to space said conducting memberswith respect to each other, said two. conducting members-having a. common point about which they are concentric, saidconducting members being: of different. dimensions, the larger enclosing the smaller, said conducting members being provided with orifices, said orifices being on a common axis whereby the cavity between said members and the cavity within the inner member may both be excited to resonance by the projection of an electron beam along the said axis by means external to the outer member.

2. The device of claim 1 the cavity enclosed Within the inner member being proportioned to be resonant at a different frequency from that at which the cavity between the two members is resonant whereby said device may be employed to emphasize the oscillation of the ultra-high frequency system at either of two frequencies.

3. The device of claim 1, the cavity enclosed within the inner member being proportioned to be resonant at the same frequency as that at which the cavity between the two members is resonant whereby said device may provide extremely effective frequency stabilization at the common resonant frequency of the two cavities.

4. In an ultra-high frequency system, a resonant device comprising solely two parallel ovoid conducting members and electroconductively insulating members having high impedance to the frequencies of the system, said last-stated members serving to space said conducting members with respect to each other, said two conducting members being concentric with respect to a common point, one conducting member enclosing the other, the conducting members being proportioned so that the cavity enclosed between them will be resonant at any one of a plurality of frequencies depending upon which of a like plurality of axes, excitation to resonance is impressed and orifices in both conducting members along a particular axis whereby the interovoid cavity will be resonant at a particular predetermined frequency upon the projection of an electron beam along said axis and the device may stabilize the frequency of the system at the said particular predetermined frequency. 

